This invention relates to an apparatus for measuring noise characteristics, and more particularly, to an apparatus which is capable of measuring a source impedance (or load impedance) for applying a minimum noise figure in the noise measurement of a transistor.
When measuring noise characteristics of a transistor at high frequency (for example, over 10 GHz), the conventional method was generally to mount a semiconductor chip on a package, and to then make the evaluation in this package state. The important factor in the noise measurement is not only the noise figure, but also the signal source and load impedances that give the minimum noise figure. For the measurement of this minimum noise figure and the source (or load) impedance to give this minimum noise figure, conventionally, it was experimentally adjusted by cutting or connecting the pattern of micro-strip line of the evaluation unit, and the evaluation unit was divided into the input side and output side, and the source and load impedances to give the minimum noise figure were measured. After dividing the evaluation unit, the connector must be connected to the circuit for measurement of impedance, and the source and load impedances varied by this connector, and hence it was impossible to make a measurement accurately. Therefore, the obtained values of source and load impedance to give the minimum noise figure contained significant errors and lacked reliability, and when an impedance conversion circuit was designed by using the obtained impedance values and connected to a transistor, problems e.g., significant noise, occurred.
These values of source and load impedances to give the minimum noise figure obtained in the package state are the values at the end of package, and contain the stray capacitance of the package and induction of the wire, and they are not exactly the impedances connected to the chip of transistor. Particularly, in a compound semiconductor, transistors and micro-strip lines are often formed on a same substrate. In this sense, it is very important to obtain the source (or load) impedance to apply the minimum noise figure as seen from the chip end of the transistor, but it was impossible to measure it in the conventional method. In the conventional method, the evaluation unit of the transistors having input and output circuits must be preliminarily separated into the input side and output side at the transistor part, and it is impossible to measure with an inseparable evaluation unit.
FIGS. 8 A, 8 B are plan and sectional views of a conventional evaluation unit for measuring the minimum noise figure and the signal source (or load) impedance to give this minimum noise figure.
In FIG. 8 A, transistor package 24 is located in the middle of the evaluation unit, and is connected to micro-strip lines 27 of the input side printed circuit board 25 and the output side printed circuit board 26. In this state, input connector 28 and output connector 29 are connected to a noise measuring system, and notch region 30 is provided in micro-strip line 27 so that the noise value may be minimized, and the impedance is adjusted while varying the pattern.
FIG. 8 B shows a sectional view taken along the line A--A' in which, as the support stand is made of copper, aluminum or other material, there are input side block 31 beneath input side printed circuit board 25, output side block 33 beneath output side printed circuit board 26, and transistor block 32 beneath transistor package 24, and they are used as the ground. These three blocks are separable, and they are mutually connected when adjusting micro-strip lines 27 to minimize the noise shown in FIG. 8 A.
FIG. 9 shows the principle of measurement for measuring the source and load impedance as seen from the transistor side after adjusting the micro-strip lines so as to minimize the noise. For the measurement of source impedance Z.sub.S and load impedance Z.sub.L, transistor block 32 shown in FIG. 8 B is separated and removed, and the impedances as seen from the transistor side of the input side and output side circuits are measured, and the source and load impedances to give the minimum noise figure are obtained.
FIG. 10 is a sectional view to show the measuring method of the source and load impedances as seen from the transistor side after adjusting the micro-strip lines. After separating and removing transistor block 32 shown in FIG. 8 B, input side block 31 on which input side printed circuit board 25 is mounted is taken out, and transistor side connector 34 is newly connected to the transistor side of input side printed circuit board 25. A 50 .OMEGA. load 18 is connected to input connector 28 of the evaluation unit, and already-calibrated measuring connector 35 having a reference plane of impedance measurement 17 is connected to transistor side connector 34, and the source impedance Z.sub.S is measured. The measuring connector 35 is linked to the impedance measuring system through a coaxial cable 8 so that the source impedance Z.sub.S may be measured. When measuring the load impedance, the same procedure as when measuring the source impedance is carried out on the output side printed circuit board 26 and the output side block 33 located beneath it.
In the conventional apparatus for measuring noise characteristics and impedance shown in FIGS. 8, 9, and 10, it is necessary for measuring characteristics to set up the transistor chips in a package and mount on the evaluation unit, and extremely complicated processes are needed, and it takes much time in measurement. Furthermore, if the pattern of the micro-strip lines is adjusted while providing a notch region in the micro-strip lines so that the noise figure may be minimum, the impedance is not varied in a wide range, and the minimum noise figure may not be always accurate. This process also takes much time in adjustment.
Moreover, in the conventional method of measuring the impedance, when measuring the source and load impedances after measuring the noise characteristics while adjusting the pattern of micro-strip lines, it is necessary to measure by cutting off the transistor block located in the middle of the evaluation unit, and it is required to compose the evaluation unit of transistors having input and output circuits so as to be separated into the input side and output side at the transistor part. Therefore, it is impossible to measure the impedance itself with an inseparable evaluation unit, or an evaluation unit to which a connector cannot be newly added. After separating the evaluation unit, if a connector can be newly connected to the input or output side for measurement of impedance, the source and load impedances vary due to the effects of the newly added connector, package leads and others, the errors increase significantly as the frequency becomes higher, thereby making it impossible to measure accurately.
Furthermore, the obtained values of source and load impedances are the values at the end of package, and are not the values at the chip end of transistor. Therefore, when composing by forming transistors and micro-strip lines on a same substrate as in a compound semiconductor device, the results of measurement could not be used, which was an extremely serious problem.